Viscoelastic fluids prepared by long chain fatty acid mixture-DMCHA ionic liquid surfactant: Two steps for recycling via pH regulation

Abstract

Ionic liquid surfactants (ILSs) show great potential for forming wormlike micelles (WLMs) due to their tunable molecular structures. However, their economic and environmental sustainability remains challenging. In this study, a fully recyclable PS-DMCHA ILSs was synthesized from a 1:1 molar mixture of palmitic acid (PA) and stearic acid (SA) (denoted PS) with N, N-dimethylcyclohexylamine (DMCHA). The HOMO, LUMO and molecular electrostatic potential were calculated by Dmol 3 with PBE functional. The formation of WLMs was confirmed through surface tension measurements, rheological analysis, and Cryo-TEM imaging. The recyclability of PS and DMCHA was validated using FTIR, ¹H NMR and molecular dynamics simulation. Results revealed that electrostatic interactions and proton transfer drive the formation of ILSs. The long carbon chain of PS contributes to the low surface tension of the ionic liquid system. The combination of PA and SA enables the formation of an ionic liquid at room temperature. When the concentration of ILSs exceeded 6.31 wt %, a wormlike network was formed. Under acidic conditions (pH<4), PS was fully precipitated from the viscoelastic fluids as a solid and separated out, while DMCHA was recovered under basic conditions (pH>12). This study offers valuable insights into enhancing both the economic and environmental feasibility of WLMs as recyclable fracturing fluid systems.